dartsim: Add support for the `<kinematic>` tag by jcmayoral · Pull Request #758 · gazebosim/gz-physics

jcmayoral · 2025-07-09T17:37:52Z

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🎉 New feature

Bring back the kinematic tag available in gazebo classic.

Summary

Adding the kinematic tag feature for dartsim physics engine. The dart joints used in gz-physics are called FreeJoints, available in dart upsteam repository in this file FreeJoint.cpp . However, as it can be observed, this joint is designed to integrate accelerations over time to calculate speed, and integrate speed to get positions. Therefore, other type of joint is needed to achieve the behavior of the Kinematic tag which ignores Forces.

In this MR for dart, an attempt to integrate this joint to the upstream repository can be found, but it seems the use case is really gazebo-related, also it will require to bump the dart version used on gz-physics. Therefore, those changes are integrated to this one MR, adding the KinematicJoint as a customization of dart (based on the feedback given on the dart MR (KinematicJoint.cc and KinematicJoint.hh).

Test it

In order to test, the MR can be compiled and using the following sdf (attached as a txt file):

kinematic-robot.sdf.txt

Once compiled, rename the file to kinematic-robot.sdf, and run

gz sim -v 4 kinematic-robot.sdf

A comparison between setting the kinematic tag (true/false) can be seen in the next video.

kinematic-tag-demo-2025-07-09_19.08.49.mp4

Checklis

Signed all commits for DCO
Added tests
Added example and/or tutorial
Updated documentation (as needed)
Updated migration guide (as needed)
Consider updating Python bindings (if the library has them)
codecheck passed (See contributing)
All tests passed (See test coverage)
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jcmayoral · 2025-07-10T01:11:28Z

Same as #756

A comparison between kinematic and non-kinematic collision with static box

kinematic-tag-demo-2025-07-09_19.08.49.mp4

azeey · 2025-08-06T21:31:33Z

See #756 (comment)

azeey · 2025-08-06T22:45:05Z

@jcmayoral can you update the PR description?

azeey · 2025-08-07T02:44:28Z

cc @iche033

jcmayoral · 2025-08-07T14:46:03Z

@azeey Is there a feature list of a issue list I can refer to? So I can complete the description?

jcmayoral · 2025-08-07T14:58:09Z

Checking the test I found that the Bazel CI was failing because of this line:

[3,384 / 3,394] 48 / 57 tests, 1 failed; Compiling dartsim/src/KinematicsFeatures.hh; 7s processwrapper-sandbox ... (4 actions running)

But I did not modify that file. Is it part of the pipeline?

azeey · 2025-08-07T16:33:36Z

@azeey Is there a feature list of a issue list I can refer to? So I can complete the description?

If there's no issue, you can skip that, but please add more description on what this PR does, and the approach you are taking. Also remove the parts of the template that are not relevant (i.e, bug fix and release sections). You can look at other PRs in this repo for reference.

jcmayoral · 2025-08-07T18:50:53Z

@azeey I just updated it. Please, let me know if something is missing.

azeey · 2025-08-20T06:09:56Z

@jcmayoral After looking at the discussion in the upstream DART PR, I tested out what setting the joint actuator type to ACCELERATION would do and I think it does make the link "kinematic". For the quickest test, I added

  joint->setActuatorType(dart::dynamics::Joint::ACCELERATION);

to

gz-physics/dartsim/src/SDFFeatures.cc

Lines 725 to 726 in d604919

	joint->setTransform(tf);

right after joint->setTransform(tf); , which makes sets the actuator type of all free joints to ACCELERATION. With this, I ran the velocity_control.sdf Gazebo example. The vehicles respond to velocity commands, but are not affected by gravity or contacts.

Screen.Recording.2025-08-20.at.00.58.24.mp4

Can you give it a try? Maybe all we need need to do is detect if a link is kinematic and set the actuator type of it's parent joint to ACCELERATION.

cc @iche033

azeey · 2025-08-22T19:58:33Z

This would be a great addition to Gazebo, but it's a large change and we're not sure about the approach taken. With
Code freeze being Monday, 08-25-2025 (see https://discourse.openrobotics.org/t/feature-freeze-for-gazebo-jetty/48187/3?u=azeey), I don't think we'll have enough time to discuss, make changes and merge this PR. Therefore, I'll remove the "Jetty Release" milestone. This means, we can't merge this until Jetty is released, however, please continue making progress on the PR and we'll try to respond as much as we can.

moriarty · 2025-08-22T20:10:13Z

@azeey it's unfortunate that this will miss the code freeze.

We originally needed the kinematic tag and had been commenting and working on top of this MR: #618 from @iche033 which adds kinematic tag for bullet but as we moved further along with our migration we realized we had behaviors which require us to use Dartsim.

What are the possibilities or options to have something like this back-ported to jetty?

azeey · 2025-08-22T20:23:38Z

What are the possibilities or options to have something like this back-ported to jetty?

It's should be possible to backport since the changes shouldn't break API/ABI. We might even be able to backport it to Ionic/Harmonic.

moriarty · 2025-08-22T20:28:59Z

Great, Thanks. We've been testing Harmonic / Ionic via source builds but if this had made it into Jetty it would have clearly given us a reason to target Jetty ASAP...

Need to get over the migration we still have performance related issues where Gazebo Classic out performs Gazebo in most cases unless you have GPUs, but this <kinematic> tag support is a key blocker and is required to work around a bunch of other things.

Signed-off-by: Jose Mayoral <jocamaba1989@gmail.com> Signed-off-by: Jose Mayoral <jcmbanos@ottomotors.com>

Signed-off-by: Jose Mayoral <jcmbanos@ottomotors.com>

dartsim/src/SDFFeatures.cc

Signed-off-by: Jose Mayoral <jcmbanos@ottomotors.com>

… into ionic-kinematic-tag

Signed-off-by: Jose Mayoral <jcmbanos@ottomotors.com>

azeey

Would you also be able to implement the KinematicLink feature which was added in #618. Not a blocker, but it would be great to have feature symmetry and be able to change the property at runtime.

azeey · 2026-03-17T17:48:55Z

dartsim/src/SDFFeatures.cc

+  bodyProperties.mInertia.setMass(sdfInertia.MassMatrix().Mass());
+  bodyProperties.mGravityMode = _sdfLink.EnableGravity();
+  bodyProperties.mInertia.setMoment(I_link);
+
+  bodyProperties.mInertia.setLocalCOM(localCom);
+  bodyProperties.mFrictionCoeff = 0;


Are any of these lines necessary? These properties are already being set above or elsewhere.

azeey · 2026-03-17T17:49:34Z

dartsim/src/SDFFeatures.cc

+  bodyProperties.mInertia.setLocalCOM(localCom);
+  bodyProperties.mFrictionCoeff = 0;
+
+  if(isKinematic){


nit: Style

Suggested change

if(isKinematic){

if(isKinematic)

{

azeey · 2026-03-17T17:50:12Z

dartsim/src/SDFFeatures.cc

+
+  if(isKinematic){
+    gzdbg << "Kinematic tag found -> " << bodyProperties.mName << std::endl;
+    jointProperties.mName = bodyProperties.mName + "_KinematicJoint";


Is there a reason for changing the name of the joint?

azeey · 2026-03-17T17:52:55Z

test/common_test/kinematic_features.cc

+              gz::math::eigen3::convert(frameData.linearVelocity));
+    EXPECT_EQ(gz::math::Vector3d::Zero,
+              gz::math::eigen3::convert(frameData.angularVelocity));
+


Can you also check that setting the velocity of the link works?

azeey · 2026-03-17T17:56:24Z

test/common_test/kinematic_features.cc

+    // currently only dartsim is supported
+    if (this->PhysicsEngineName(name) != "dartsim")
+      continue;


Bullet-featherstone also supports kinematic links (see #618).

jcmayoral requested review from azeey and scpeters as code owners July 9, 2025 17:37

github-actions bot added the 🏛️ ionic Gazebo Ionic label Jul 9, 2025

azeey added this to Core development Jul 9, 2025

github-project-automation bot moved this to Inbox in Core development Jul 9, 2025

jcmayoral force-pushed the ionic-kinematic-tag branch from 311f5f1 to a39f1d0 Compare July 24, 2025 21:08

azeey added this to the Jetty Release milestone Jul 28, 2025

azeey removed this from the Jetty Release milestone Aug 6, 2025

jcmayoral changed the base branch from gz-physics8 to main August 6, 2025 22:15

jcmayoral changed the title ~~Kinematic Tag for DartSim (Ionic)~~ Kinematic Tag for DartSim (Main) Aug 6, 2025

jcmayoral mentioned this pull request Aug 6, 2025

Kinematic Tag for Dartsim (Harmonic) #756

Closed

26 tasks

azeey added 🪵 jetty Gazebo Jetty and removed 🏛️ ionic Gazebo Ionic labels Aug 6, 2025

azeey added this to the Jetty Release milestone Aug 6, 2025

azeey moved this from Inbox to In review in Core development Aug 6, 2025

azeey changed the title ~~Kinematic Tag for DartSim (Main)~~ dartsim: Add support for the <kinematic> tag Aug 6, 2025

azeey removed this from the Jetty Release milestone Aug 22, 2025

jcmayoral and others added 20 commits March 4, 2026 08:52

test just 1d velocity

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Signed-off-by: Jose Mayoral <jocamaba1989@gmail.com> Signed-off-by: Jose Mayoral <jcmbanos@ottomotors.com>

rename variable

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fix ci pipeline

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missing correction for pipeline

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MERGE?

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update MR

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remove not changed files

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eofile space add tutorials-2

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fix messages in sdffeature

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remove differences

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remove duplicated SetKinematicFeaturesList

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jcmayoral force-pushed the ionic-kinematic-tag branch from 96a8c91 to fd18910 Compare March 4, 2026 14:53

Merge branch 'main' into ionic-kinematic-tag

3594b1a

azeey reviewed Mar 10, 2026

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Jose Mayoral and others added 6 commits March 11, 2026 12:00

replace gravity with actuatortype

e2566ae

Signed-off-by: Jose Mayoral <jcmbanos@ottomotors.com>

Merge branch 'ionic-kinematic-tag' of github.com:jcmayoral/gz-physics…

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… into ionic-kinematic-tag

remove ament_ignore

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Merge branch 'main' into ionic-kinematic-tag

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Merge branch 'main' into ionic-kinematic-tag

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Merge branch 'main' into ionic-kinematic-tag

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azeey requested changes Mar 17, 2026

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🎉 New feature

Summary

Test it

Checklis

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